This invention relates to a method of bleaching limestone, and more particularly to a method for removing small amounts of iron, usually in the form of iron oxide, from the limestone in order to improve the whiteness of the limestone.
Most limestone deposits contain small amounts of iron oxides or other forms of iron. The iron imparts a yellow, reddish, or pink tint to the limestone rendering the limestone, even when ground, totally unacceptable for many uses, and therefore of little commercial value. The most lucrative filler and pigment applications require a bright white ground limestone product with little or no hue or tint. Very few limestone deposits are naturally white enough for the most lucrative markets.
In order to produce a white limestone product, most limestone ore requires some form of color beneficiation. If the iron oxide is in a form that is discrete from the limestone, the iron oxide can first be treated by grinding the ore fine enough to liberate the iron oxide. Then the free iron oxide may be removed physically by several known processes, such as screening or classification, magnetic separation, or flotation. The iron oxide may also be removed from the ground limestone by chemically bleaching or leaching with solvents for iron oxides followed by filtering. Dithionite-type bleaching agents perform most efficiently under acidic conditions of 2 to 5 pH, which cannot be obtained due to buffering by the calcium carbonate in the limestone as it reacts with the acid.
One method for chemically removing iron from limestone is taught in the U.S. Pat. No. 4,239,736, to Fenske now owned by Franklin Industries, Inc., the common assignee of this application. The patented Fenske process includes the mixing of ground limestone with water to form a slurry to which is added a flocculating agent, such as acetic acid, and a bleaching agent, such as sodium hydrosulfite. This process is carried out at a temperature preferably between 70-120 deg. F. for a sufficient length of time to allow the iron oxide to undergo a chemical reaction with the bleaching agent, the resultant iron product passing into solution with the water. The slurry is then filtered to physically remove the dissolved iron from the limestone, leaving the limestone with an improved whiteness.
However, it has been found that the Fenske process is primarily successful on grades of limestone that are nearly white enough naturally to be sold as white products without chemical treatment to beneficiate the color. It has been found that only small amounts of iron are removed by the Fenske process, while the poorer grades of limestone with more iron oxide contaminants still retain an undesirable tint.
As noted in the prior Fenske U.S. Pat. No. 4,239,736, bleaching agents other than sodium hydrosulfite, such as zinc hydrosulfite, K-Brite - 3E, and K-Brite - 3K of the Virginia Chemical Company, as well as sulfur dioxide and zinc dust can also be used in the chemical reaction with iron, and particularly iron oxide.
Another method of increasing the whiteness of limestone by optically transforming the natural tint of the limestone by introducing a pigment of a different color is disclosed in the common assignee's U.S. Pat. No. 4,661,164, issued Apr. 28, 1987.